Nucleic acid purification method

ABSTRACT

Disclosed is a process for separating and/or purifying a nucleic acid by elution of the nucleic acid from anion exchange resins under conditions of high salt concentration and the presence in the eluant of an additive comprising guanidine, or a guanidine-like derivative. The process allows high recovery of nucleic acids from anion exchange resins without impairing the nucleic acid stability as compared with conventional ion exchange chromatographic procedures.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. provisional patent applicationNo. 60/826,916 filed Sep. 26, 2006; the entire disclosure of which isincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

This invention relates generally to methods for separating nucleic acidssuch as genomic DNA, plasmid DNA and mRNA from contaminating cellularcomponents such as proteins, lipids, soluble membrane components and thelike. In particular, the invention relates to the improved recovery ofnucleic acids from anion exchange chromatography media in either batchor packed mode by the addition of guanidine or guanidine-likederivatives in the presence of salt.

BACKGROUND OF THE INVENTION

The last three decades has seen considerable effort in the developmentof improved methods for the isolation and purification of nucleic acidsfrom biological sources. This has been due mainly to the increasingapplications of nucleic acids in the medical and biological sciences.Genomic DNA isolated from blood, tissue or cultured cells has severalapplications, which include PCR, sequencing, genotyping, hybridizationand southern blotting. Plasmid DNA has been utilized in sequencing, PCR,in the development of vaccines and in gene therapy. Isolated RNA has avariety of downstream applications, including blot hybridization, invitro translation, cDNA synthesis and RT-PCR.

The analysis and in vitro manipulation of nucleic acids is typicallypreceded by a nucleic acid isolation step in order to free the nucleicacid from unwanted contaminants which may interfere with subsequentprocessing procedures. For the vast majority of procedures in bothresearch and diagnostic molecular biology, extracted nucleic acids arerequired as the first step. In a typical DNA extraction protocol, cellsor homogenized tissue samples containing the nucleic acid of interestare harvested and lysed using standard methods, for example usingenzymes such as Proteinase K and lysozyme; detergents, such as SDS,Brij, Triton X100, or using other chemicals such as sodium hydroxide,guanidium isothiocyanate, etc. (See for example, Sambrook et al,Molecular Cloning—A Laboratory Manual 2nd edition 9.14 (New York: ColdSpring Harbor Laboratory 1989). Following removal of the cellulardebris, the crude lysate is treated with organic solvents such asphenol/chloroform to extract proteins. RNA may be removed or reduced ifrequired by treatment of the enzymes such as RNAse. However, thepresence of contaminants such as salts, phenol, detergents and the likecan interfere with many downstream manipulations for which the nucleicacid is intended.

Currently several procedures are available for the chromatographicpurification of DNA (genomic and plasmid) and RNA, for example, byemploying silica based membrane purification, size exclusionchromatography, reversed phase chromatography, gel filtration, magneticbead based purification, or ion-exchange chromatography. Ion exchangechromatography is one of the most commonly used separation andpurification methods and has been used for purification of plasmid DNA,genomic DNA and RNA. See for example, U.S. Pat. No. 6,410,274(Bhikhabhai), U.S. Pat. No. 6,310,199 (Smith et al), U.S. Pat. No.6,090,288 (Berlund et al), U.S. Pat. No. 5,990,301 (Colpan et al), U.S.Pat. No. 5,856,192, U.S. Pat. No. 5,866,428 (Bloch), U.S. Pat. No.5,801,237 (Johansson), EP 1125943 b1 (Macherey, Nagel GmbH & Co) EP992583 B1, EP 616639 (Qiagen), U.S. Pat. No. 5,707,812, U.S. Pat. No.5,561,064 (Vical Inc.).

While anion exchange chromatographic procedures for the purification ofnucleic acids have been extensively referenced, one of the shortcomingsof current protocols is the impaired recovery of nucleic acid during theelution step, (Endres, H. N. et al, Biotechnol. Appl. Biochem., (2003),37(3), 259-66; Prazeres, D. M. et al, J. Chromatog. A. (1998), 806(1),31-45; Urthaler J. et al, Acta Biochim. Pol., (2005), 52(3), 703-11;Ferreira, G. N. et al, Bioseparation, (2000), 9(1), 1-6; Ferreira, G.N., et al, Biotechnol. Prog., (2000), 16(3), 416-24. Addition of organicagents such as polyols and alcohols during adsorption and desorption hasbeen shown to improve selectivity and recovery during anion exchangepurification of DNA (Tseng, W. C. et al, J. Chromatogr. B Analyt.Technol. Biomed. Life Sci., (2003), 791(1-2), 263-72). However, thereappear to be no reports that specifically address the recovery issuesoften seen during DNA desorption from anion exchange resins. The presentinvention addresses this problem since it relates to improvingrecoveries of bound DNA from anion exchange resins. In particular, theinvention allows improved desorption of the DNA from the solid supportwithout further manipulation of the protocol.

Plasmid DNA, genomic DNA and RNA have similar charge properties to oneanother and are polyanions having high charge density. Binding topositively charged ion exchange resins is therefore possible in thepresence of up to 0.7M sodium chloride, depending on the length andconformation of the nucleic acid to be adsorbed. An increase in nucleicacid length as well as double stranded conformation results in anincrease in binding strength between the nucleic acid and the anionexchanger. However, this effect is only proportional to nucleic acidlength up to about 2 kilobases. The very strong interaction between thenegatively charged phosphate backbone of the nucleic acid and ionexchange resin hampers elution of the nucleic acid using conventionalmethods, where a simple increase in ionic strength of the salt eluant issufficient for recovery of 70-100% of the bound material. However, inthe case of longer chain nucleic acids, an increase in ionic strength upto 3M salt only allows recoveries of 20-50% of the bound nucleic acid.Recovery of the remaining bound material can be accomplished with acombination of high salt concentration and elevated pH using sodiumhydroxide. However, sodium hydroxide is not only caustic, but may alsolead to irreversible denaturation of nucleic acids and degradation overtime.

BRIEF DESCRIPTION OF THE INVENTION

Surprisingly, it has now been found that nucleic acids can beefficiently eluted from anion exchange resins under conditions of highsalt concentration and the presence in the eluant of an additivecomprising guanidine, or a guanidine-like derivative, the pH of theeluant being suitably in the range of about pH 9 to about pH 13, andmore preferably in the range of about pH 10.5 to about pH 11.6. Theaddition of guanidine or a guanidine-like derivative to the elutionsolution has been shown to improve recovery of nucleic acids from anionexchange resins from 20-50% to 70-95%.

Thus, in a first aspect the present invention provides a process forseparating and/or purifying a nucleic acid, comprising:

a) contacting an aqueous solution containing the nucleic acid with ananion exchanger bound to a solid support matrix under conditions suchthat the anion exchanger binds the nucleic acid; andb) eluting the anion exchanger with an aqueous mobile phase comprising anucleic acid elution salt solution;characterised in that the elution solution comprises an additive havingthe formula (I):

wherein:R is a moiety selected from the group consisting of hydrogen, a loweralkyl, optionally substituted by amino, and the group:

where n is 1, 2 or 3;wherein the presence of the additive in the elution solution provides anincrease in the recovery of nucleic acid from the anion exchanger, ascompared with the recovery of the nucleic acid from the same anionexchanger and in the absence of the additive in the elution solution,all other conditions being equal.

Thus, the present invention provides a method for the use of a compoundas an additive to the elution solution to allow high recovery of nucleicacids from anion exchange resins without impairing the nucleic acidstability as compared with conventional ion exchange chromatographicprocedures. Nucleic acids, consist of a chain (or a paired chain ofdeoxyribose phosphate monomers covalently linked via phospho-diesterbonds, each sugar phosphate moiety carrying a single aromaticheterocyclic base: adenine (A), guanine (G), cytosine (C), thymine (T)found solely in DNA), and uracil (U) found solely in RNA). In aqueoussolutions of a pH>2, the highly soluble hydrophilic sugar-phosphatepolymer backbone contributes one negative charge for eachphospho-diester group, with the exception of the terminalphospho-monoester, which may carry up to two negative charges. DNA isthus a polyanion, where the net negative charge of the nucleic acidmolecule is related directly to chain length. Nucleic acids therefore,display strong binding affinities to anion exchange resins such that ahigh salt concentration in the elution solution is required toefficiently remove the nucleic acid from the resin. Compounds such asguanidine, guanidine-like compounds such as methyl- and ethyl-guanidine,and compounds containing a guanidinium moiety such as arginine) mediateinteraction with nucleic acids through electrostatic interactions andhydrogen bonding.

The positive impact of guanidine and guanidine-like compoundsparticularly arginine on recovery of nucleic acids from anion exchangeresins is most pronounced at alkaline pH, for example, between about pH9 and about pH 13, more particularly at pH values between about 10 and12. A pH range of between about 10.5 and 11.6 appears to provide optimumrecovery. Recovery of nucleic acids is highest when arginine orguanidine carbonate is added to the elution solution. Without beingbound by theory, one of the unique properties of the guanidinium groupis its delocalized positive charge property which is due to theconjugation between the double bond and the nitrogen lone pairs. Theguanidinium group is able to form multiple hydrogen-bonds preferentiallywith guanine bases, which may act to cause local deformation of thenucleic acid structure, which change in conformation of nucleic acidscontributes to a change in desorption kinetics thereby favouring highrecoveries of nucleic acids during anion exchange chromatography.Macromolecules are known to bind to adsorptive surfaces through multiplepoints of attachment creating microenvironments on chromatography mediasurfaces, which allow adsorption that can become close to irreversiblewith conventional desorption techniques. While traditional anionexchange chromatography allows the elution of bound molecule from thepositively charged ligand through an increase in competing salt anions,it has been observed that elution of nucleic acids, especially highmolecular weight (HMW) nucleic acids (above 0.1 kilobases), is notefficient with salt anions alone. It has also been observed that thecation used as a counter ion, as well as the pH of elution has an effecton recovery of HMW nucleic acids, the use of strongly alkaline (such aswith sodium hydroxide) may be detrimental to recovery because of theco-elution of contaminants and detrimental effects on product stability.

In one embodiment, the additive is a compound having the formula (I)

(and more particularly the carbonate or bicarbonate salt thereof), whereR is selected from H, and lower alkyl, optionally substituted by amino.Suitably, lower alkyl is a C₁ to C₄ alkyl group, for example methyl,ethyl, propyl and butyl, preferably, methyl or ethyl. Where R is anamino-substituted lower alkyl group, examples of the compounds accordingto formula (I) include 2-aminoethyl-guanidine, 3-aminopropyl-guanidineand 4-aminobutyl-guanidine (agmatine). In a particularly preferredembodiment, R is hydrogen, thus compound (I) is guanidine, as itscarbonate or bicarbonate salt.

In a second embodiment, the additive is a compound having the formula(I):

wherein R is the group:

where n is 1, 2 or 3,preferably 3. In this case, the additive is arginine, suitablyL-arginine, D-arginine, or a mixture of both optical isomers.

In embodiments according to the invention, it is preferred that theguanidine and guanidine-like compound is present as an additive in theelution solution at a concentration of between 0.1M and 2M, preferablybetween 0.25M and 0.5M. The elution solution will typically comprise asalt solution, suitably between about 0.7M and 3M to which the additiveis added. Suitably, the pH of the aqueous mobile phase is between aboutpH 9 and about pH 13, the preferred range of pH being between about pH10 and about pH 12, more preferably between about pH 10.5 and about pH11.6.

The term “nucleic acid” as used herein refers to any DNA or RNAmolecule, or a DNA/RNA hybrid, or mixtures of DNA and/or RNA. The term“nucleic acid” therefore is intended to include genomic or chromosomalDNA, plasmid DNA, amplified DNA, total RNA and mRNA. The processaccording to the present invention is particularly suitable for thepreparation and/or purification of genomic DNA derived from complexmixtures of components derived from cellular and tissue samples from anyrecognised source, including normal and transformed cells, with respectto species (e.g. human, rodent, simian), tissue source (e.g. brain,liver, lung, heart, kidney skin, muscle) and cell type (e.g. epithelial,endothelial, blood); or for the purification of plasmid DNA derived fromE. coli and yeast.

Furthermore, the present method is suitable for the preparation and/orpurification of genomic DNA having a size of from about 0.1 kilo-basesto about 200 kilo-bases, or of plasmid DNA, cosmid, BAC or YAC DNA. Thepresent invention is useful for purifying plasmid DNA and cosmid DNA, inparticular for downstream applications in molecular biological research,such as cloning and sequencing, gene therapy and in diagnosticapplications both in vivo and in vitro.

Anion exchange resins suitable for use with methods of the presentinvention include both strong anion exchangers and weak anionexchangers, wherein the anion exchange resin suitably comprises asupport carrier to which charged or chargeable groups have beenattached. The ion exchange resin may take the form of a bead, a membraneor a surface. Examples of strong anion exchange resins includeQ-Sepharose fast flow resin, Q-Sepharose XL and CaptoQ. Examples of weakion exchange resins include ANX fast flow resin and DEAE Sephadex A-25resin (GE Healthcare).

By employing an additive of formula (I) in the aqueous mobile phase itis possible to increase recovery of nucleic acid from the anionexchanger of at least 40% and typically between about 40% and about400%, as compared with the recovery of said nucleic acid from the sameanion exchanger and in the absence of the additive in the elutionsolution, all other conditions being equal.

Other features and advantages of the invention will be apparent from thefollowing description of the preferred embodiments, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows pulse-field gel electrophoresis analysis of purifiedgenomic DNA from human blood samples, using a method according to oneembodiment of the invention.

FIG. 2 shows an agarose gel image of purified genomic DNA from humanblood samples.

FIG. 3 shows comparison of restriction enzyme (EcoRI) digested andun-digested genomic DNA that was purified from human blood samples.

FIG. 4 shows real time PCR amplification results obtained from thegenomic DNA samples from human blood, with very similar amplificationprofiles observed among the samples.

FIG. 5 shows pulse-field gel electrophoresis analysis of purifiedgenomic DNA from rat liver samples, using a method according to oneembodiment of the invention.

FIG. 6 shows real time PCR amplification results obtained from thegenomic DNA samples from rat liver samples, with very similaramplification profiles observed among the samples.

FIG. 7 shows comparison of restriction enzyme (HindIII) digested andun-digested genomic DNA that was purified from rat liver samples.

FIG. 8 shows pulse-field gel electrophoresis analysis of purifiedgenomic DNA from cultured MRC5 cells samples, using a method accordingto one embodiment of the invention.

FIG. 9 shows comparison of restriction enzyme (EcoRI) digested andun-digested genomic DNA that was purified from MRC5 cells samples.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following examples serve to illustrate the DNA purificationprocesses according to embodiments of the present invention and are notintended to be limiting.

(A) Protocols Used in the Examples

(a) Isolation of Genomic DNA from Blood

Genomic DNA isolation from blood is done in 2 steps. The first step isthe lysis of blood and the second step is purification of genomic DNAusing ion-exchange column chromatography.

Lysis: This process involved 2 steps. First white blood cells areisolated and then the isolated white blood cells are lysed using lysissolution. The protocol used for the isolation of white blood cells andlysis of white blood cells is as follows. Five ml of blood is used as anexample here. However, the protocol can be adjusted accordingly based onthe amount of blood used.

1. Add 5 ml of whole blood to a 50 ml conical centrifuge tube.

2. Add 5 ml of pre-chilled Lysis1 solution and 15 ml of chilled water tothe sample. Place the tubes in a rack and mix well by inverting thetubes 10-15 times.

3. Incubate at ambient temperature for 10 minutes. 4. Centrifuge at1500×g for 10 minutes. 5. Discard the supernatant into a waste containercontaining diluted bleach solution (or follow appropriate safetyprecautions as recommended by the EHS) without disturbing pellet. 6. Add1 ml of Lysis1 solution and 3 ml of water to the centrifuge tube andre-suspend the pellet by vortexing briefly. 7. Centrifuge at 5000×g for10 minutes. 8. Discard the supernatant carefully without disturbing thewhite blood cell pellet. 9. Re-suspend the white blood cell pellet in 5ml of Lysis2 solution by vortexing at highest speed for 30 sec to 1 min.10. Add 50 μl of Proteinase K (20 mg/ml) (AG Scientific), vortex brieflyand incubate at ambient temperature for 20 minutes. 11. Add 5 ml ofLoading Solution to the centrifuge tube and mix well by swirling thetube and load this solution on the purification column. Purification:The purification process also includes a de-salting process (steps17-21).

12. Remove the cap from the top of an ion exchange purification column(approximately 1.5 ml of ion exchange resin in a plastic tube, packedusing an automated process). Discard the solution by decanting. Cut theclosed end of the column at the notch and place the columns in 50 mlcentrifuge tubes using column adaptors.

13. Transfer the lysis solution obtained from step 11 above to thecolumn and allow it to flow completely through the resin by gravity. 14.Apply 5 ml of Loading Solution to the column. 15. When all the LoadingSolution passes through the resin, place the columns in fresh 50 mlcentrifuge tubes. 16. Add 2.5 ml of Elution Solution to the column andcollect the product in the eluate. Desalting:

17. Remove the cap of desalting column and discard the solution. Cut theclosed end of the column at notch and place the column in a centrifugetube using the adaptor.

18. Equilibrate the column by applying 25 ml of 1×TE buffer (10 mMTris-HCl, pH 8.0, 1 mM EDTA). This can be accomplished by using LabMatePD-10 buffer reservoir (GE Healthcare) in one step. 19. Transfer theeluate (2.5 ml) from the purification step 16 to desalting column andallow it to flow by gravity. 20. When the solution has completelyentered the gel bed, place the column in fresh 50 ml centrifuge tube.21. Add 3.5 ml of 1×TE buffer to each column and collect the eluatecontaining genomic DNA. The desalted samples are now ready forquantitation and downstream applications.

(b) Isolation of Genomic DNA from Tissue Samples

The tissue sample is prepared by the following steps. It is critical tohave a completely homogenized sample to obtain good yield of genomic DNAfrom the purification process.

1. Weigh approximately 100 mg of tissue by slicing into very finepieces.

2. Wash the tissue with 1×PBS buffer. Add 1 ml of 1×PBS buffer, vortexand centrifuge at 1000 RPM for 1 min. Discard the washing and remove anytraces of buffer left in the tube using a pipette.

3. Add 0.5 ml of 1×PBS buffer and homogenize the sample by handheldhomogenizer. Tissue samples so prepared are subjected to the followingsteps for the isolation of genomic DNA. Steps 4-7 are for sample lysis;steps 8-12 are for purification; and steps 13-17 are for de-salting. 4.Add 0.5 ml of Lysis Solution to the homogenized sample (PBS and lysisSolution in 1:1 ratio) and vortex at the highest possible speed for20-30 sec. 5. Add 50 μl of proteinase K (20 mg/ml) solution, vortexbriefly and incubate at 60° C. for 1.5 hours.

6. After the incubation period, cool the reaction tubes in an ice bathfor 3 min. Add 20 μl of RNAse A solution (20 mg/ml) and incubate at 37°C. for 15 min.

7. Dilute the crude lysate with 4 ml of DNAse free water and 5 ml ofLoading Solution and centrifuge at 5000×g for 15 min to pelletparticulates. Purification:

8. Remove the cap from the top of purification column. Discard thesolution by decanting. Cut the closed end of the column at the notch andplace the columns in 50 ml centrifuge tubes using column adaptors.

9. Transfer the lysis solution to the column and allow it to flowcompletely through the resin by gravity. 10. Apply 5 ml of LoadingSolution to the column. 11. When all the Loading Solution passes throughthe resin, place the columns in fresh 50 ml centrifuge tubes. 12. Add2.5 ml of Elution Solution to the column and collect the product in theeluate. Desalting:

13. Remove the cap of desalting column and discard the solution. Cut theclosed end of the column at notch and place the column in a centrifugetube using the adaptor.14. Equilibrate the column by applying 25 ml of 1×TE buffer. This can beaccomplished by using LabMate PD-10 buffer reservoir.

15. Transfer the eluate (2.5 ml) from the purification step 12 todesalting column and allow it to flow by gravity. 16. When the solutioncompletely entered the gel bed, place the column in fresh 50 mlcentrifuge tube. 17. Add 3.5 ml of 1×TE buffer to each column andcollect the eluate containing genomic DNA. The desalted samples are nowready for quantitation and downstream applications.

(c) Isolation of Genomic DNA from Cell Cultures

Cell cultured cells are collected and lysed according to the protocolbelow. The purification and desalting is done as described in protocol(b) (“Isolation of genomic DNA from tissue samples”) above.

1. Wash between 1×10⁷ and up to 2.0×10⁷ cells with 1×PBS buffer (2×5ml). Suspend the cells in 5 ml of 1×PBS buffer and centrifuge at 2000×gfor 10 min. Decant the buffer carefully from the pellet and repeat theprocess once more.

2. Re-suspend the cell pellet completely in 1 ml of 1×TE Buffer byvortexing for 30 seconds to 1 minute. 3. Add 4.5 ml of Lysis Solutionand vortex for 15-30 sec. 4. Add 50 μl of Proteinase K (20 mg/ml) andvortex briefly (2 sec). 5. Incubate at 60° C. for 1-2 hours. 6. Cool thetube in an ice bath for 2 min and add 20 ul of RNase A (20 mg/ml). 7.Incubate at 37° C. for 15 min.

(d) Quantitation of the Purified Genomic DNA Samples

Quantitation of the purified genomic DNA samples was achieved with a UVspectrophotometer, using 1×TE Buffer pH8.0 as the blank and 1 cm pathlength cuvettes. Readings of three samples were taken at A260, A280 andA320. Yield of DNA (μg)=A260×50 μg×Eluted sample volume (3.5 ml).

(e) Detailed Composition of Solutions Used in the Protocols(i) Blood gDNA Protocol:

-   -   Lysis1 solution: 30 mM Tris-HCl, 10 mM Magnesium chloride, 2%        Triton X 100 and 0.6M sucrose.    -   Lysis2 solution: 20 mM Tris-HCl, 20 mM EDTA, 20 mM sodium        chloride and 0.1% SDS.    -   Loading solution: 700 mM sodium chloride, 50 mM Tris and 1 mM        EDTA.        (ii) Tissue Protocol:    -   Lysis solution: 20 mM Tris-HCl, 20 mM EDTA, 100 mM sodium        chloride and 1% SDS.    -   Loading solution: 700 mM sodium chloride, 50 mM Tris and 1 mM        EDTA.        (iii) Cell Culture Protocol:    -   Lysis Solution: 20 mM Tris-HCl, 20 mM EDTA, 100 mM sodium        chloride and 1% SDS.

(B) Evaluation of Various Solutions for Genomic DNA Elution

In an effort to find an optimal elution solution for genomic DNApurification, high ionic salt strength solutions were tested for elutionof genomic DNA from the bound anion exchanger.

As an example, ANX Sepharose fast flow (high sub) resins were used here.These resins are very stable over a wide pH range (3-13), with anaverage particle size of 90 μm (We have subsequently tested other anionexchange resins and found they work well too). The columns werepre-packed using a salt solution having similar strength as the sampleloading solution, with suitable anti-microbial agent (e.g. ethanol orkethon). This eliminates the need for column equilibration prior toloading of the sample in loading solution for binding of the nucleicacid.

The source cell used here was human blood and the blood protocoldescribed in (A) (a) above was followed. We found that even when a saltconcentration of 2 to 3M was used it did not significantly improve therecovery of genomic DNA from the ion-exchange columns. After elution allthe samples were desalted using NAP-10 or NAP-25 columns (GE Healthcare)and the DNA was quantified using UV spectrophotometer. To identify abuffer or a solution which could provide better recovery, differentcombinations of salts were evaluated.

The following elution buffer/solution combinations were evaluated forgenomic DNA elution in individual experiments.

1. 50 mM Tris-HCl, pH 8.0, 1 mM EDTA, 200 mM NaCl2. 50 mM Tris-HCl, pH 8.0, 1 mM EDTA, 200 mM NaCl+0.2 M sodium carbonate3. 50 mM Tris-HCl, pH 8.0, 1 mM EDTA, 200 mM NaCl+0.2 M sodiumperchlorate4. 50 mM Tris-HCl, pH 8.0, 1 mM EDTA, 200 mM NaCl+0.2 M sodiumbicarbonate5. 50 mM Tris-HCl, pH 8.0, 1 mM EDTA, 200 mM NaCl+0.2 M magnesiumchloride

6. 2M Sodium iodide 7. 2M Sodium perchlorate 8. 3M Ammonium acetate 9.3M Ammonium acetate+0.2M sodium bicarbonate 10. 3M Ammonium acetate+0.2Msodium carbonate 11. 3M Ammonium acetate+0.2M sodium biborate 12. 3MAmmonium bicarbonate 13. 3M Sodium bicarbonate (not dissolvedcompletely) 14. 3M Sodium carbonate 15. 3M Sodium phosphate (notdissolved completely, precipitates)

16. 50 mM Tris-HCl, pH 8.0, 1 mM EDTA, 200 mM NaCl+25 mM sodiumhydroxide17. 50 mM Tris-HCl, pH 8.0, 1 mM EDTA, 200 mM NaCl+50 mM sodiumhydroxide18. 50 mM Tris-HCl, pH 8.0, 1 mM EDTA, 200 mM NaCl+75 mM sodiumhydroxide19. 50 mM Tris-HCl, pH 8.0, 1 mM EDTA, 200 mM NaCl+100 mM sodiumhydroxide20. 50 mM Tris-HCl, pH 8.0, 1 mM EDTA, 200 mM NaCl+75 mM Lithiumhydroxide

21. 2M Salt+500 mM L-Arginine 22. 1M sodium chloride+1M sodium carbonate

Representative results are shown below in Table 1. From the data it isclear that 2M salt solution itself elutes less than half of the genomicDNA that could be eluted with a combination of salt and sodiumhydroxide, or salt and arginine.

TABLE 1 Representative elution solution combinations and genomic DNAyields Eluant Yield Purity 2M Salt buffer 32 μg 1.80 2M Salt buffer + 50mM NaOH 72 μg 1.87 2M Salt buffer + 75 mM NaOH 79 μg 1.89 2M Saltbuffer + 75 mM LiOH 76 μg 1.92 2M Salt buffer + 100 mM NaOH 72 μg 1.882M Salt buffer 500 mM NaOH 74 μg 1.8 2M Salt + 1M Sodium carbonate 56 μg1.76

Recovery of the remaining bound material can be accomplished with acombination of high salt concentration and elevated pH using sodiumhydroxide. However, sodium hydroxide is not only caustic, but may alsolead to irreversible denaturation of nucleic acids and degradation overtime.

It has also been observed that the cation used as a counter ion, as wellas the pH of elution has an effect on recovery of HMW nucleic acids, theuse of strongly alkaline (such as with sodium hydroxide) may bedetrimental to recovery because of the co-elution of contaminants anddetrimental effects on product stability.

Since arginine, which has a carboxylic acid group and also a guanidiniumgroup, was showing a dramatic effect in the elution process, weevaluated other amino acids and guanidinium salts and also thecombination of amino acid and guanidinium salt to identify whether it isthe carboxylic acid group or guanidinium group that contributes to theimproved recovery. The results are summarized in Table 2.

TABLE 2 Evaluation of various amino acids and guanidinium salts fortheir effects in improving genomic DNA elution Eluant Yield Purity 3MSalt buffer 37 μg 1.78 2M Salt buffer + 1M Sodium carbonate 62 μg 1.732M Salt buffer + 0.5M Arginine 82 μg 1.80 2M Salt buffer + 0.1M Arginine46 μg 1.78 2M Salt buffer + 0.5M Aspartic acid 39 μg 1.63 2M Saltbuffer + 0.5M Aspartic acid + 0.5M 41 μg 1.57 Guanidine HCl 2M Saltbuffer + 0.5M Guanidine 39 μg 1.64 2M Salt buffer 50 mM NaOH 76 μg 1.81

We noticed that the elution solutions which improve genomic DNA recoveryand elution have a common feature, that is an elevated pH of around 10.5to 11.5. It appears that improved elution of nucleic acids employingguanidinium is facilitated by the presence in the elution solution ofcarbonate (or bicarbonate). Based on this observation furtherexperiments were performed to test the effect of elevated pH. Wecompared 2M sodium chloride with sodium carbonate or 2M sodium chloridewith Tris base or arginine. The results obtained are given in Table 3.

TABLE 3 Evaluation of various solutions with similar pH for theireffects on improving genomic DNA elution Eluant Yield Purity 2M Saltbuffer 64 μg 1.81 2M Salt buffer + 0.25M Arginine 110 μg  1.81 2M Saltbuffer + 0.5M Arginine 118 μg  1.82 2M Salt buffer + 0.5M Sodiumcarbonate 93 μg 1.79 2M Salt buffer + 1M Sodium carbonate 93 μg 1.74 2MSalt buffer + 0.5M Tris base 86 μg 1.77 2M Salt buffer + 1M Tris base 85μg 1.77 2M Salt buffer + 50 mM NaOH 118 μg  1.82 2M Salt buffer + 0.5MArginine 101 μg  1.82 2M Salt buffer + 0.5M Arginine 97 μg 1.82

Based on the evaluation of several different solutions pH appears toplay a critical role in the recovery of genomic DNA in addition to thesalt strength. Combination of 1 to 2M sodium chloride, with 0.25M to0.5M arginine, 0.5-1M sodium carbonate and 0.5-1M Tris can be used forimproved elution.

Since elevated pH appeared to be the factor that helped recover higheramount of genomic DNA from the ion-exchange resins, several morecombinations of salt with guanidine derivatives were evaluated as anelution solution, all of which provided higher pH for the elutionsolution.

1. 2M NaCl+0.2 M L-Arginine 2. 2M NaCl+0.5M Guanidine carbonate 3. 2MNaCl+0.5M Guanidine carbonate+0.5M Glycine 4. 2M NaCl+0.5M Guanidinecarbonate+0.5M L-glutamic acid 5. 2M NaCl+0.5M Guanidine propionic acid

The results clearly demonstrate that addition of arginine, guanidinecarbonate or other guanidine derivatives such guanidine propionic acidto 1M to 2M sodium chloride solution has similar effects on the elutionof genomic DNA from the ion-exchange resins (Table 4).

TABLE 4 Evaluation of additional solutions with similar pH for theireffects in improving genomic DNA elution Eluant Yield Purity 2M Saltbuffer + 0.25M Arginine 121 μg 1.8 2M Salt buffer + 0.5M Guanidinecarbonate 128 μg 1.8 2M Salt buffer + 0.5M Guanidine carbonate + 0.5M130 μg 1.81 Glycine 2M Salt buffer + 0.5M Guanidine carbonate + 0.5M 125μg 1.79 L-Glutamic acid 2M Salt buffer + 0.5M Guanidine propionic acid126 μg 1.77

(C) Comparison of L-Arginine, Guanidine Carbonate and PotassiumCarbonate in Elution Solution

Since a combination of sodium chloride and sodium carbonate did providesome improvement in the recovery, a solution of potassium carbonate incombination with sodium chloride is evaluated, in comparison withguanidine carbonate and L-arginine. The pH of a solution of sodiumchloride and sodium carbonate is not optimal for complete recovery ofthe nucleic acids from the ion-exchange resin. Since potassium carbonatewill give higher pH solution, it is expected to give higher recovery ofthe nucleic acids from ion-exchange resins. Indeed a combination ofsodium chloride and potassium carbonate did give a solution with higherpH and the elution profile compared well with a solution containingguanidine carbonate or arginine. The experimental details are similar tothose of section (B) supra. Again, human blood was used as the genomicDNA source and the blood protocol described in (A) (a) was followed. Theresults are shown in Table 5.

TABLE 5 Evaluation of the effect of L-Arginine, Guanidine Carbonate andPotassium Carbonate in elution solutions Eluant Yield Purity 1M NaCl +0.5M L-Arginine 146 1.77 1M NaCl + 0.5M Guanidine carbonate 139 1.77 1MNaCl + 0.5M Potassium carbonate 144 1.79

From the data in Table 5, it is clear that a sodium chloride solutioncontaining potassium carbonate is equally effective in genomic DNAelution, when compared with a solution containing either guanidinecarbonate or L-arginine, from ion-exchange resins. Based on thesystematic evaluation of various additives, pH appears to play acritical role in the recovery of nucleic acid in addition to the saltstrength. Combination of 1 to 2M sodium chloride, with 0.25 to 0.5Marginine, 0.5M potassium carbonate or 0.5M guanidine carbonate can beused for improved elution. 0.5 to 1M sodium carbonate or 0.5M to 1M Trisbase can also be used to increase the elution as well.

(D) Genomic DNA Purification from Blood

An 8 ml sample of human blood was lysed using the procedure described inthe protocol section. The crude lysate was diluted with loading solutionand loaded on the ion-exchange purification column. After all thesolution passed through the resin an additional 5 ml of loading solutionwas added onto the column. When there was no more solution on the top ofthe resin, 2.5 ml of elution solution (1M sodium chloride+0.5M potassiumcarbonate) was added and the eluate containing genomic DNA was collectedin a collection tube. The product obtained was desalted using NAP-25column. The size of the genomic DNA isolated was determined by PulseField Gel Electrophoresis (FIG. 1). The purity of the product wasassessed by UV spectrophotometry and by gel analysis (FIG. 2). Thegenomic DNA obtained by this method was also evaluated in downstreamapplications such as restriction digestion (FIG. 3), Multiplex PCR andReal Time PCR (FIG. 4).

By Pulse Field Gel Electrophoresis, it is clear that the purifiedgenomic DNA from Blood is of large size (FIG. 1). The purity of thesample was examined by an agarose gel analysis (FIG. 2). It demonstratesthat the genomic DNA isolated is pure and without RNA contamination.

The quality of the purified genomic DNA was assessed by several methods.

The DNA was subjected to restriction enzyme digest using EcoRI. Purifiedgenomic DNA (250 ug) was digested with 40 units of the enzyme. Thedigested sample was analyzed on an agarose gel side-by-side withun-digested sample DNA. The gel image shows that all the genomic DNA wascompletely digested (FIG. 3, Lanes 2, 4, 6 represent the purified,un-digested genomic DNA, while Lanes 1, 3, 5 are samples digested withthe enzyme).

The quality of the genomic DNA samples was indirectly measured by theefficiency in a multiplex PCR reaction. A long range multiplex PCR forthe P450 genes were used for this test (CodeLink P450 protocol, GEHealthcare). Three amplicons from genes CYP2D6, CYP3A4, and CYP3A5 wereamplified in a single reaction. The size in amplicons ranges from 335 bpto 2600 bp. The size and yield of the PCR products were determined viathe Agilent Biolanalyzer 2100 and DNA 7500 kit. The multiplex PCRreactions worked well for all the samples tested (data not shown).

The quality of the genomic DNA samples was also tested by real time PCRassays. Real time PCR experiments were done using Applied Biosystems7900HT Fast Real Time PCR System. All the samples tested show verysimilar amplication profiles (FIG. 4).

The same purification process has been successfully applied to bloodsamples from other animals as well. High quality genomic DNA wasisolated from different animals such as rat, Guinea pig, horse, chickenand sheep.

(E) Isolation of Genomic DNA from Tissue Samples

Two hundred milligrams of rat liver tissue was homogenized and lysed asdescribed in the protocol section. The crude lysate was diluted withloading solution and centrifuged to pellet any particulates. The clearlysate was loaded on the ion-exchange purification column. After all thesolution passed through the resin, 5 ml of loading solution was added tothe column. When there was no more solution left on the top of theresin, 2.5 ml of elution solution (1M sodium chloride+0.5M potassiumcarbonate) was added to the column and the product was collected in theeluate. The genomic DNA thus obtained was desalted using NAP-25 column.The purity of the product was assessed by UV spectrophotometry and bygel analysis. Multiple samples were processed to access the consistencyof the protocol. The size of the genomic DNA isolated was determined byPulse Field Gel Electrophoresis (FIG. 5). The genomic DNA obtained bythis method was also evaluated in downstream applications such as realtime PCR (FIG. 6), and restriction digestion (FIG. 7).

By Pulse Field Gel Electrophoresis, it is clear that all the purifiedgenomic DNA samples from rat liver tissue are of large size (FIG. 5).The purity of the sample was examined by an agarose gel analysis. Itdemonstrated that the genomic DNA isolated is pure and without RNAcontamination (data not shown).

The quality of the purified genomic DNA was assessed by several methods.

The quality of the genomic DNA samples was tested by real time PCRassays. Real time PCR experiments were performed using AppliedBiosystems 7900HT Fast Real Time PCR System. All the samples tested showvery similar amplication profiles (FIG. 6).

The DNA was subjected to restriction enzyme digest using HindIII.Purified genomic DNA (250 ug) was digested with 40 units of the enzyme.The digested sample was analyzed on an agarose gel side-by-side withun-digested sample DNA. The gel image shows that all the genomic DNA wascompletely digested (FIG. 7, Lanes 2, 4, 6, 8 represent the purified,un-digested genomic DNA, while Lanes 1, 3, 5, 7 are samples digestedwith the enzyme).

(F) Isolation of Genomic DNA from Cell Cultures

Approximately 2×10⁷ MRC5 cells were lysed using the procedure describedin the protocols section. The crude lysate was diluted with loadingsolution and transferred to the ion-exchange purification column. Afterall the solution passed through the resin, 5 ml of loading solution wasadded to the column. When there was no more solution left on the top ofthe resin, 2.5 ml of elution solution was added to the column and theproduct was collected in the eluate. The genomic DNA thus obtained wasdesalted using NAP-25 column. The purity of the product was assessed byUV spectrophotometry and by gel analysis. The size of the genomic DNAisolated was determined by Pulse-Field Gel Electrophoresis. The genomicDNA obtained by this method was also evaluated in downstreamapplications such as real time PCR and restriction digestion.

By Pulse Field Gel Electrophoresis, it is clear that all the purifiedgenomic DNA samples from MRC5 cells are of large size (100 Kb; FIG. 8).The purity of the sample was examined by an agarose gel analysis. Itdemonstrated that the genomic DNA isolated is pure and without RNAcontamination (data not shown).

The quality of the purified genomic DNA was assessed by restrictiondigest. The DNA was subjected to restriction enzyme digest using EcoRI.Purified genomic DNA (250 ug) was digested with 40 units of the enzyme.The digested sample was analyzed on an agarose gel side-by-side withun-digested sample DNA. The gel image shows that all the genomic DNA wascompletely digested (FIG. 9, Lanes 2, 4, 6, 8, 10, 12 represent thepurified, un-digested genomic DNA, while Lanes 1, 3, 5, 7, 9, 11 aresamples digested with the enzyme).

The present invention provides a kit for the separation and/orpurification of a nucleic acid from a cellular sample, the kitcomprising a lysis solution for generating an aqueous solutioncontaining the nucleic acid from the; an anion exchanger bound to asolid support matrix for binding the nucleic acid; an elution solutionfor eluting the nucleic acid from the anion exchanger; and optionallydesalting means for desalting said eluted nucleic acid. Suitably thereis present in the elution solution an additive having the formula (I):

wherein:

R is a moiety selected from the group consisting of hydrogen, a loweralkyl,

optionally substituted by amino, and the group:

where n is 1, 2 or 3.

In one embodiment, the additive is arginine. In another embodiment theadditive is guanidine present as its carbonate or bicarbonate salt.

Suitably, the anion exchanger is ANX fast flow resin. Alternatively theanion exchanger is DEAE Sephadex A-25 resin, Q-Sepharose fast flowresin, or Q-Sepharose XL, or CaptoQ resin.

All patents, patent publications, and other published referencesmentioned herein are hereby incorporated by reference in theirentireties as if each had been individually and specificallyincorporated by reference herein. While preferred illustrativeembodiments of the present invention are described, one skilled in theart will appreciate that the present invention can be practiced by otherthan the described embodiments, which are presented for purposes ofillustration only and not by way of limitation. The present invention islimited only by the claims that follow.

1. A process for separating and/or purifying a nucleic acid, comprising:a) contacting an aqueous solution containing the nucleic acid with ananion exchanger bound to a solid support matrix under conditions suchthat the anion exchanger binds the nucleic acid; and b) eluting theanion exchanger with an aqueous mobile phase comprising a nucleic acidelution salt solution; wherein the elution solution comprises anadditive having the formula (I):

wherein: R is a moiety selected from the group consisting of hydrogen, alower alkyl, optionally substituted by amino, and the group:

where n is 1, 2 or 3; wherein the presence of the additive in theelution solution provides an increase in the recovery of nucleic acidfrom the anion exchanger, as compared with the recovery of the nucleicacid from the same anion exchanger and in the absence of the additive inthe elution solution, all other conditions being equal.
 2. The processof claim 1, wherein R is selected from H, and lower alkyl.
 3. Theprocess of claim 2, wherein lower alkyl is a C₁ to C₄ alkyl group,preferably methyl or ethyl.
 4. The process of claim 1, wherein saidadditive is present as its carbonate or bicarbonate salt.
 5. The processof claim 1, wherein said additive is guanidine, as its carbonate orbicarbonate salt.
 6. The process of claim 1, wherein R is the group:

where n is 1, 2 or
 3. 7. The process of claim 1, wherein said additiveis arginine.
 8. The process of claim 1, wherein said additive is presentin the aqueous mobile phase at a concentration of between 0.1M and 2M.9. The process of claim 8, wherein said additive is present in theaqueous mobile phase at a concentration of between 0.25M and 0.5M. 10.The process of claim 8, wherein said additive is present in the aqueousmobile phase at a concentration of about 0.5M.
 11. The process of claim1, wherein the pH of said aqueous mobile phase is between about pH 10and about pH
 13. 12. The process of claim 11, wherein the pH of saidaqueous mobile phase is between about pH 10.5 and about pH 11.6.
 13. Theprocess of claim 1, wherein the presence of said additive in the aqueousmobile phase provides an increase in the recovery of nucleic acid fromthe anion exchanger of at least 40%, as compared with the recovery ofsaid nucleic acid from the same anion exchanger and in the absence ofsaid additive in said elution solution, all other conditions beingequal.
 14. The process of claim 1, wherein the presence of said additivein the aqueous mobile phase provides an increase in the recovery ofnucleic acid from the anion exchanger of between about 40% and about400%, as compared with the recovery of said nucleic acid from the sameanion exchanger and in the absence of additive in said elution solution,all other conditions being equal.
 15. The process of claim 1, whereinthe nucleic acid is genomic DNA.
 16. The process of claim 1, wherein thenucleic acid is plasmid DNA.
 17. The process of claim 1, wherein thenucleic acid is RNA.
 18. The method of claim 1, wherein the anionexchanger is ANX fast flow resin.
 19. The method of claim 1, wherein theanion exchanger is selected from the group consisting of DEAE-SephadexA-25 resin, Q-Sepharose fast flow resin, Q-Sepharose XL resin and CaptoQresin.
 20. A kit for the separation and/or purification of a nucleicacid from a cellular sample, said kit comprising: a) a lysis solutionfor generating an aqueous solution containing the nucleic acid from saidcellular sample; b) an anion exchanger bound to a solid support matrixfor binding the nucleic acid; c) an elution solution for eluting thenucleic acid from said anion exchanger; and d) desalting means fordesalting said eluted nucleic acid; wherein there is present in saidelution solution an additive having the formula (I):

wherein: R is a moiety selected from the group consisting of hydrogen, alower alkyl, optionally substituted by amino, and the group:

where n is 1, 2 or
 3. 21. The kit of claim 20, wherein the additive isarginine.
 22. The kit of claim 20, wherein the additive is guanidinecarbonate or bicarbonate salt.
 23. The kit of claim 20, wherein theanion exchanger is ANX fast flow resin.
 24. The kit of claim 20, whereinthe anion exchanger is selected from DEAE-Sephadex A25 resin,Q-Sepharose fast flow resin, Q-Sepharose XL resin and CaptoQ resin.